A fractional-N PLL with small ΔK vco wideband LC-VCO and current-matching CP for M-DTV systems

Abstract

An Σ-Δ fractional-N frequency synthesiser with small K vco-variation wideband LC voltage controlled oscillator (LC–VCO) and current-matching charge pump (CP) for Mobile Digital television Systems is presented. To achieve small VCO-gain (K vco) variation, a parallel switched varactor array is proposed to the conventional wideband LC-VCO with switched capacitor array, the value of the switched varactor is pre-set and both arrays are controlled by the same switching code. Perfect current matching and good stability are obtained by the improved CP with an added bias branch circuit for low reference spur. The chip was fabricated in a Taiwan Semiconductor Manufacturing Company (TSMC) 0.25 µm complementary metal-oxide-semiconductor process and draws 12 mA from a 2.5 V supply voltage. The synthesiser covers a wide tuning range from 0.82 to 1.85 GHz with two integrated LC-VCOs, and each VCO achieves a K vco variation of less than 16% with a tuning range of more than 46%. The current mismatch of CP is as low as 1.2%. The measured close-in and out-of-band phase noise are −83.5 dBc/Hz@10 kHz and −127 dBc/Hz@1 MHz, respectively, the reference spur is −76.3 dBc.